Based on new insights into the molecular processes of ALI we hypothesize that a longitudinal analysis of cells of innate immunity and thrombosis will demonstrate changes in RNA and protein expression that are significantly associated with progression of All from sepsis. Hypothesis 1: The progression of sepsis-induced ALI is significantly associated with critical interactions between Th1 and Th2 lymphocytes and platelets. Specific Aim 1. We will collect total RNA from peripheral blood neutrophils, monocytes, TH1 and TH2 and CD8+ T lymphocytes and platelets from 20 mechanically ventilated pediatric patients with early ALI from sepsis. Blood sampling will take place at 6 individual time points over 72 hours along with severity of illness scoring to determine degree of disease progression. The temporal series of all 6 cell types banked for 3 retrospectively identified patients exemplary of ALI progression will be expression profiled and compared to profiles in 3 non-progressive ALI patients and 3 normal controls in order to generate a map of potential cell-cell interactions. Hypothesis 2: The pattern of peripheral blood cell expression of genes and proteins functionally important to the progression of ALI from sepsis will reflect the pattern of gene and protein expression in pulmonary edema cells and fluid. Specific Aim 2. Using the prioritization model from Aim 1, we will select the 2 cell types that show promise as "indicator" cell types. From the prospective cell collections in Aim 1 and more focused collections in this aim, we will choose, at random, 15 progressive and 15 non-progressive ALI subjects as determined by criteria outlined in the text of this proposal. We will verify select indicator gene expression changes found in Aim 1 by QMF RT-PCR in all 30 subjects (e.g. in the 20 genes with the most significant fold changes or most functional relevance.) Functionally important changes in gene expression will be confirmed in the entire cohort on the protein level by flow cytometry, and/or ELISA of proteins in peripheral blood and pulmonary edema fluid.